New testing method for the evaluation of topographical gradients of varying strength

Passive gradient motion found in nature is becoming a point of interest for heat exchange and green energy technologies. Surfaces with a topographical gradient could potentially enhance heat exchanger performance, promote dropwise vs filmwise condensation, or delay icing on wind turbines by assisting in the removal of condensed or impacted droplets. Coating-free topographical surface tension gradients can be fabricated via various methods and need to be examined in terms of their capability for spontaneous droplet motion. In this work, a simple experimental method, coupled with numerical modeling and force analysis, for examining variable-pitch micro/nanoengineered hierarchical superhydrophobic gradients is shown. The method was validated against numerical calculations, allowing the strength of the gradients to be compared. In most cases, model predictions for droplet travel distance and velocity were within 20% of the measured data. This method could also be useful for gradient design improvements in the absence of spontaneous motion on a horizontal surface.